Vehicular rearview mirror assemblies have proven to be a convenient location for providing drivers with useful information. For example, a video display disposed behind a rearview mirror, but visible through a portion of the mirror, may supply the driver with a video image of the scene to the rear of the vehicle where the driver's view may otherwise be obstructed. Similarly, an information display may offer the driver such vehicle-telemetry information as vehicle speed, engine status, oil level and temperature, for example, or any other information of interest. Integration of backup displays or other displays behind the mirror element in an automotive rearview mirror assembly is generally preferred over placing them adjacent to the mirror element, that would increase the area of the overall mirror assembly and impair the driver's view through the windshield.
Displays that have been, or might be, used in automotive applications employ various principles such as vacuum fluorescence (VF), electromechanics (EM), light emitting or organic light emitting diodes (LED or OLED), plasma display panels (PDP), microelectromechanical systems (MEMS), electroluminescence (EL), projection (the projection systems include but are not limited to DLP and LCOS), or liquid crystal technology (used in liquid crystal displays, or LCDs), to name just a few. High-resolution LCDs capable of delivering color images, for example, may be mass-produced reliably and at low cost. LCDs are also noteworthy in that the liquid crystal medium changes its polarizing properties under the influence of the applied electric field and the light emanating from an LCD is polarized.
LCDs used in applications such as LCD-based television or a computer display are conventionally illuminated with light emitters that light up the LCD from the side or back of the display panel. A conventional type of emitters used with LCDs has been a cold cathode fluorescent lamp (CCFL). Increasingly, however, light-emitting diodes (LEDs) are becoming the predominant backlighting technology of choice. Recent advances in LED technology, packaging, and materials have provided dramatic increases in LED brightness which, in turn, has led to the use of the LED in all types of lighting applications, including LCD backlights. Over the last several years, small color back-lit LCDs have been integrated into an ever-broadening range of products, including vehicular rearview mirror assemblies that utilize auto-dimming electro-optical mirrors or regular prismatic mirrors. A related light source subsystem for illumination of a display in a rearview automotive mirror typically includes an array of high-brightness white LEDs emitting light in the direction of the LCD, a diffuser that spatially redistributes the emitted light on its way to the LCD, and an electronic driver that provides a regulated constant current to drive the LEDs. The size of the display will often determine the number of LEDs required for illuminating of the LCD. In at least one embodiment the LEDs have a minimal shift in color over drive current, thereby, the LEDs are powered via an analog signal to reduce the amount of radiated emissions.
One of challenges presented by the use of display technology in an automotive context is that of providing the driver with preferably spatially uniform luminance sufficient to see the display clearly, especially under daunting conditions of ambient light, while, at the same time, providing a clear and undistorted reflected view of the rear and peripheral scene to the driver. Since automotive reflectors serve a crucial safety function in identifying objects otherwise outside of the driver's field of view, they must critically preserve image quality.
Applications illustrating various types of automotive mirror displays are disclosed in commonly assigned U.S. Pat. Nos. 6,870,655; 6,737,630; 6,572,233; 6,552,326; 6,420,800; 6,407,468; 6,346,698; 6,170,956; 5,883,605; and 5,825,527, U.S. patent application Ser. No. 12/367,143 entitled “A Vehicular Rearview Mirror Assembly Including Integrated Backlighting for a Liquid Crystal Display (LCD),” U.S. patent application Ser. No. 12/193,426 entitled “Vehicle Rearview Assembly Including A Display for Displaying Video Captured by a Camera and User Instructions,” U.S. patent application Ser. No. 12/196,476 entitled “Discrete LED Backlight Control for a Reduced Power LCD Display System,” and U.S. patent application Ser. No. 12/570,585 entitled “Multi-display mirror system and method for expanded view around the vehicle.” Various types of displays used in conjunction with the rearview automotive mirrors are known in the art such as alphanumeric displays, graphical displays, video displays such as rear camera display (RCD), and combinations thereof. These displays are discussed, for example, in U.S. Pat. No. 7,221,363, and in U.S. Patent Publications Nos. 2008/0068520. Aggregately, the above-mentioned patent documents are referred to hereinafter as “Our Prior Applications”. The disclosure of each of abovementioned patent documents is incorporated herein by reference in its entirety.